This invention relates to electric hammers, in particular rotary hammers, having an air cushion hammering mechanism.
Such hammers will normally have a housing and a hollow cylindrical spindle mounted in the housing. The spindle allows insertion of the shank of a tool or bit, for example a drill bit or a chisel bit, into the front end thereof so that it is retained in the front end of the spindle with a degree of axial movement. The spindle may be a single cylindrical part or may be made of two or more co-axial cylindrical parts, which together form the hammer spindle. For example, a front part of the spindle may be formed as a separate tool holder body for retaining the tool or bit. Such hammers are provided with an impact mechanism which converts the rotational drive from an electric motor to a reciprocating drive causing a piston, which may be a hollow piston, to reciprocate within the spindle. The piston reciprocatingly drives a ram by means of a closed air cushion located between the piston and the ram. The impacts from the ram are transmitted to the tool or bit of the hammer, optionally via a beatpiece.
Such hammers can also be employed in combination impact and drilling mode or in a drilling only mode in which the spindle, or a forwardmost part of the spindle, and hence the bit inserted therein will be caused to rotate. In the combination impact and drilling mode the bit will be caused to rotate at the same time as the bit receives repeated impacts. A rotary drive mechanism transmits rotary drive from the electric motor to the spindle to cause the spindle, or a forwardmost part thereof to rotate.
In smaller hammers, a wobble drive arrangement is generally used to convert a rotary drive from the motor to the reciprocating drive of the piston. In a known arrangement the rotary drive from the motor is transmitted to an intermediate shaft mounted within the hammer housing generally parallel to the axis of the spindle. A wobble sleeve is rotatably mounted on the intermediate shaft. The wobble sleeve is formed with a wobble race which extends around the wobble sleeve at an oblique angle to the axis of the intermediate shaft. Balls are set to run between this inner race and an outer race of a wobble ring, which wobble ring has a wobble pin extending from it to the rearward end of the piston. The wobble pin is pivotally connected to the rearward end of the piston via a trunnion arrangement. Thus, when the wobble sleeve is rotatably driven the wobble pin reciprocates and reciprocatingly drives the piston within the spindle and hammering occurs. In drilling only mode hammering is not required and so a mode change mechanism is required to selectively transmit the rotation of the intermediate shaft to the wobble sleeve.
It is known to have a mode change element moveable along the intermediate shaft in a first direction in order to be engaged with sets of teeth on the wobble sleeve and the intermediate shaft to actuate hammering or in a second opposite direction in order to be disengaged with one of the sets of teeth to disable hammering. The mode change element generally requires some means of determining its end positions on the intermediate shaft. This is generally provided by an axial stop element mounted on the intermediate shaft or the wobble sleeve using a circlip. Such axial stops and circlips are difficult to assemble, if they are not assembled correctly the hammer will not operate correctly and if they become loose, then they can damage other components of the hammer. Alternatively, a mode change linkage, connected to a mode change knob or the mode change knob itself, which act to move the mode change element between its different positions can be used to determine the end positions of the mode change element. However, this may reduce the accuracy with which the end positions can be determined and so may lead to a less compact design.
In smaller hammers, where the compactness of the hammer is a critical design issue, the mode change mechanism must be compact. However, the mode change mechanism must also be robust so that it can operate reliably in the high vibration environment of a hammer.